In most animal species including humans, commissural axons connect neurons on the left and right side of the nervous system. This communication between the two sides of the brain and spinal cord is necessary for a series of complex function, including binocular vision, coordinated locomotor movements, and sound direction localization. In humans, abnormal axon midline crossing during development causes a whole range of neurological disorders ranging from congenital mirror movements, horizontal gaze palsy, scoliosis or binocular vision deficits. We are using genetic strategies to perturb and rewire commissural connections in mice and analyze the consequence on behavior and brain function. The mechanisms which guide axons across the CNS midline were thought to be evolutionary conserved but our recent results suggest that they differ across vertebrates. We have identified a receptor that might have played a key role in the development of mammalian specific circuits. To facilitate the analysis of brain connectivity in mouse mutants, we have also developed a new imaging method that combines tissue clearing and light-sheet fluorescent microscopy.